Direct Electrical Heating of a Flexible Pipe

Abstract

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 26936, “Direct Electrical Heating of a Flexible Pipe,” by T. Holst, T. Larsen, A. Straarup, K. Glejbol, and K.S. Olsen, National Oilwell Varco Denmark, prepared for the 2016 Offshore Technology Conference, Houston, 2–5 May. The paper has not been peer reviewed. Copyright 2016 Offshore Technology Conference. Reproduced by permission. This paper describes a new direct electrical heating method in flexible-pipe systems. The method effectively expands the operational envelope of flexible unbonded pipes by offering prevention and removal of hydrate plugs and mitigation of wax precipitation by distributed heating along the entire pipe length. The direct-electrical-heating solution is effectively operated as a coaxial cable, where electrical current is passed through the carcass and returned through the tensile-armor (TA) layers. The Concept of Direct-Heated Flexible Pipes The general structure of the flexible unbonded pipe is shown in Fig. 1. The inside of the pipe consists of a stainless, wound carcass surrounded by a polymeric pressure sheath defining the fluid barrier. On top of the liner is pressure armoring made from interlocked C-shaped profiles. TA is made from two layers of helically wound steel profiles wound in opposite directions. Thermal-insulation tape may be present on top of the TA. The pipe is protected by a polymeric outer sheath. The idea behind the new flexible-pipe- heating concept is to pass electrical current directly through the existing carcass structure and return the current through the TA wires. This configuration makes it possible to feed the power into the pipe with the electrical feeds being connected only to the one end of the pipe, which is normally mounted topside. This electrical configuration resembles a standard, coaxial-cable geometry, where the polymeric pressure sheath serves as the dielectric insulator. Although the direct-heating solution can be used as a standalone system, there are significant advantages to combining it with a fiber-optic thermal- monitoring system. Together, these two technologies yield a complete heating system that allows for monitoring and control.